Syntheses and photophysical investigations of Cr(III) hexadentate iminopyridine complexes and their tris(bidentate) analogues.

We report the preparation, photophysical characterization, and computed excited state energies for a family of Cr(III) complexes based on iminopyridine (impy) Schiff base ligands: compounds 1 and 2 feature hexadentate ligands where tren (tris-(2-aminoethyl)amine) caps three impy groups; compounds 3 and 4 are tris(bidentate) analogues of 1 and 2; compounds 2 and 4 contain methyl ester substituents to alter ligand donation properties relative to 1 and 3, respectively. Cyclic voltammograms exhibit multiple reversible ligand-based reductions; the hexadentate and tris(bidentate) analogues have almost identical reduction potentials, and the addition of ester substituents shifts reduction potentials by +200 mV. The absorption spectra of the hexadentate complexes show improved absorption of visible light compared to the tris(bidentate) analogues. Over periods of several hours to days, the complexes undergo ligand-substitution-based decomposition in 1 M HCl((aq)) and acetonitrile. For freshly prepared sample solutions in CH(3)CN, time-resolved emission and transient absorption measurements for 4 show a doublet excited state with 17-19 µs lifetime at room temperature, while no emission or transient absorption signals from the doublet states are observed for the hexadentate analogue 2 under the same conditions. The electronic structure contributions to the differences in observed photophysical properties are compared by extensive computational analyses (UB3LYP MD-DFT and TD-DFT-NTO). These studies indicate that the presence of nonligated bridgehead nitrogen atoms for 1 and 2 significantly reduce excited state doublet, quartet, and sextet energies and change the character of the low lying doublet states in comparison to species that show population of doublet excited states.

Structural and electronic comparison of 1st row transition metal complexes of a tripodal iminopyridine ligand.

We report the preparation and characterization of a series of divalent 3d transition metal complexes (Cr to Zn, 1-7), featuring the multidentate, tripodal iminopyridine Schiff-base ligand trimethyl 6,6',6″-((1E,1'E,1″E)-((nitrilotris(ethane-2,1-diyl))tris(azanylylidene))tris-(methanylylidene))trinicotinate (L(5-OOMe)). X-ray structural studies carried out on 1-5 and 7 reveal complex geometries ranging from local octahedral coordination to significant distortion toward trigonal prismatic geometry to heptacoordinate environments. Regardless of coordination mode, magnetic and spectroscopic studies show the ligand to provide moderately strong ligand fields: the Fe complex is low-spin, while the Co and Mn complexes are high-spin at all temperatures probed. Cyclic voltammograms exhibit multiple reversible ligand-based reductions, which are relatively consistent throughout the series; however, the electrochemical behavior of the Cr complex 1 is fundamentally different from those of the other complexes. Time-dependent (TD) density functional theory (DFT) and natural transition orbital (NTO) computational analyses are presented for the ligand, its anion, and complexes 1-7: the computed spectra reproduce the major differential features of the observed visible absorption spectra, and NTOs provide viable interpretations for the observed features. The combined studies indicate that all complexes contain neutral ligands bound to M(II) ions, except for the Cr complex 1, which is best described as a Cr(III) species bound to a radical anionic ligand.

Synthesis and solution phase characterization of strongly photooxidizing heteroleptic Cr(III) tris-dipyridyl complexes.

We report the preparation and characterization of Cr(III) coordination complexes featuring the dimethyl 2,2'-bipyridine-4,4'-dicarboxylate (4-dmcbpy) ligand: [(phen)(2)Cr(4-dmcbpy)](OTf)(3) (1), [(Ph(2)phen)(2)Cr(4-dmcbpy)](OTf)(3) (4), [(Me(2)bpy)(2)Cr(4-dmcbpy)](OTf)(3) (7), and [Cr(4-dmcbpy)(3)](BF(4))(3) (8), where phen is 1,10-phenanthroline, Ph(2)phen is 4,7-diphenyl-1,10-phenanthroline, and Me(2)bpy is 4,4'-dimethyl-2,2'-bipyridine. Static and nanosecond time-resolved absorption and emission properties of these complexes dissolved in acidic aqueous (1 M HCl) solutions are reported. Emission spectra collected at 297 K show a narrow spectrum with an emission maximum ranging from 732 nm (1) to 742 nm (4). The emissive state is thermally activated and decays via first order kinetics at all temperatures explored (283 to 353 K). At 297 K the observed lifetime ranges from 7.7 micros (8) to 108 micros (4). The photophysical data suggest that in these acidic aqueous environments these complexes store approximately 1.7 eV for multiple microseconds at room temperature. Of the heteroleptic species, complex 4 shows the greatest absorption of visible wavelengths (epsilon = 1270 M(-1) cm(-1) at 491 nm), and homoleptic complex 8 has improved absorption at visible wavelengths over [Cr(bpy)(3)](3+). The electrochemical properties of 1, 4, 7, and 8 were investigated by cyclic voltammetry. It is found that inclusion of 4-dmcbpy shifts the "Cr(III/II)" E(1/2) by +0.22 V compared to those of homoleptic parent complexes, with the first reduction event occurring at -0.26 V versus Fc(+)/Fc for 8. The electrochemical and photophysical data allow for excited state potentials to be determined: for 8, Cr(III*/II) lies at +1.44 V versus ferrocenium/ferrocene (approximately +2 V vs NHE), placing it among the most powerful photooxidants reported.

Anion dependence in the spin-crossover properties of a Fe(II) podand complex.

We report the syntheses, characterisations, and spin state behaviours of salts of the tripodal-ligated Fe(II) complex [FeL(6-OH)]X(2) (L(6-OH) = tris{4-[(6-methanol)-2-pyridyl]-3-aza-3-butenyl}amine, X = OTf(-) (1), Br(-) (2), I(-) (3), BPh(4)(-) (4)). Covalent linking of the ligand arms is imperative as a high-spin bis(tridentate) complex (5) is formed when a non-tethered ethyl iminopyridine ligand (L(2) = 4-[(6-methanol)-2-pyridyl]-3-aza-3-butenyl) is used. For salts 1-4, thermally-induced spin-crossover (SCO) is observed in the solid state, with dependence on anion and solvate molecules. Salts with larger anions show more complete SCO centred at higher temperatures (1 > 3 > 2); the triflate salt 1 (T(1/2) = 173 K) also shows the strongest cooperativity of the compounds examined. Hydrogen bonding appears to be critical to SCO in this family of salts: limiting interactions by use of tetraphenylborate produces a high-spin complex down to 5 K. In protic solvents such as methanol, spectra of [FeL(6-OH)](2+) are largely unchanged over a period of three days, but dissociate when interrogated with strong field bidentate ligands. Compounds 1-3, and 5 remain high spin in solution down to 180 K, consistent with the data obtained in the solid state.